Hydroclimate variability in the Nile River Basin during the past 28,000 years

Abstract

13It has long been known that extreme changes in North African hydroclimate occurred 14 during the late Pleistocene yet many discrepancies exist between sites regarding the timing, 15 duration and abruptness of events such as Heinrich Stadial (HS) 1 and the African Humid 16Period (AHP). The hydroclimate history of the Nile River is of particular interest due to its 17 lengthy human occupation history yet there are presently few continuous archives from the 18 Nile River corridor, and pre-Holocene studies are rare. Here we present new organic and 19 inorganic geochemical records of Nile Basin hydroclimate from an eastern Mediterranean 20 (EM) Sea sediment core spanning the past 28 ka BP. Our multi-proxy records reflect the 21 fluctuating inputs of Blue Nile versus White Nile material to the EM Sea in response to 22 2 gradual changes in local insolation and also capture abrupt hydroclimate events driven by 23 remote climate forcings, such as HS1. We find strong evidence for extreme aridity within 24 the Nile Basin evolving in two distinct phases during HS1, from 17.5 to 16 ka BP and from 25 16-14.5 ka BP, whereas peak wet conditions during the AHP are observed from 9-7 ka BP. 26We find that zonal movements of the Congo Air Boundary (CAB), and associated shifts in 27 the dominant moisture source (Atlantic versus Indian Ocean moisture) to the Nile Basin, 28 likely contributed to abrupt hydroclimate variability in northern East Africa during HS1 29 and the AHP as well as to non-linear behavior of hydroclimate proxies. We note that 30 different proxies show variable gradual and abrupt responses to individual hydroclimate 31 events, and thus might have different inherent sensitivities, which may be a factor partially 32 contributing to the controversy surrounding the abruptness of past events such as the AHP. Introduction 40The paleoclimate history of the Nile River valley in East Africa is of interest due to its 41 rich history of human occupation (Vermeersch and Van Neer, 2015). Relationships 42 between climate and the distribution of settlements on the Nile River corridor have long 43 been recognized and it is hypothesized that extreme changes in African hydroclimate 44 helped shape the growth and led to the decline of numerous complex societies [Kuper and 45 3 Kröpelin, 2006]. One of the most dramatic changes in North African hydroclimate, the so-46 called African Humid Period (AHP) [deMenocal et al., 2000], occurred during the early 47Holocene when increased rainfall allowed vegetation, lakes and human populations to 48 occupy a "green Sahara", a region that today is a hyperarid desert [Kuper and Kröpelin, 49 2006]. Variability in Nile River flow also played an important role in shaping Egypt's 50 civilizations with the collapse of the Old Kingdom at 4,160 years before present, attributed 51 to a 30 year absence of annual Nile flooding [Stanley et al., 2003]. Although lacking direct 52 evidence, it is hypothesized that HS1, which is recognized as an extreme and widespread 53 drought in North Afri...

Humans as Agents in the Termination of the African Humid Period

“…Ecological boundary conditions in regions affected by the termination of the HCO have been attributed to their position relative to the Intertropical Convergence Zone (ITCZ)-once the rain belt migrated southwards, their predominant source of moisture was substantially reduced (Liu et al, 2007;Castañeda et al, 2016). As will be detailed below, the speed with which the transition from high-to low-biodiversity landscapes occurs is variable according to both longitude and latitude.…”

“…Atlantic Ocean sea surface temperatures also contributed to West African monsoon dynamics during the middle Holocene (Weldeab et al, 2005), but do not appear to have impacted local vegetation patterns as significantly as in northern Africa. Tierney et al (2011) and Tierney and deMenocal (2013) attribute rapid termination of the AHP in northeastern Africa to constriction of the Congo Air Boundary (CAB) and ITCZ during the middle Holocene (see also Costa et al, 2014;Junginger et al, 2014;Bloszies et al, 2015;Castañeda et al, 2016). The CAB and ITCZ previously trapped tropical Atlantic moisture into a much wider area within the greater Sahara and Sahel region during the AHP, but with weakening summer insolation during the termination phase, the eastern and northern extents of these pressure convergence zones constricted between 5 • and 7 • relative to their maximum geographic extents (Figure 2; Gasse and Roberts, 2004).…”

The C<sub>32</sub> alkane-1,15-diol as a proxy of late Quaternary riverine input in coastal margins

“…This change also affects the soil input into the Nile River, as inferred from the distribution of branched GDGTs, with a more arid climate reducing the vegetation in the Ethiopian Highlands (source of the Blue Nile) and favoring soil erosion, while during a more humid climate, vegetation increases and soil erosion is less (Krom et al, 2002). To summarize, the climate of the Nile catchment area was colder and drier (Castañeda et al, 2010(Castañeda et al, , 2016 during the YD, H1 and the LGM. The LGM and H1 were extremely arid events with the likely desiccation of the Nile water sources, i.e., Lake Tana and Lake Victoria (Castañeda et al, 2016).…”

“…Associated H1 and LGM cooling, extreme aridity in the Nile catchment is observed as inferred from the δD of leaf waxes. In contrast, during the early Holocene S1 deposition, a more humid climate and enhanced Nile River runoff prevailed (Castañeda et al, 2016). Neodymium (ε Nd ) and strontium ( 87 Sr / 88 Sr) isotopes (Castañeda et al, 2016, andBox et al, 2011, respectively) show a relative increase in the contribution of Blue Nile inputs when the climate is arid (H1, LGM) and an increased contribution of the White Nile inputs when the climate is humid (S1).…”

“…In contrast, during the early Holocene S1 deposition, a more humid climate and enhanced Nile River runoff prevailed (Castañeda et al, 2016). Neodymium (ε Nd ) and strontium ( 87 Sr / 88 Sr) isotopes (Castañeda et al, 2016, andBox et al, 2011, respectively) show a relative increase in the contribution of Blue Nile inputs when the climate is arid (H1, LGM) and an increased contribution of the White Nile inputs when the climate is humid (S1). This change also affects the soil input into the Nile River, as inferred from the distribution of branched GDGTs, with a more arid climate reducing the vegetation in the Ethiopian Highlands (source of the Blue Nile) and favoring soil erosion, while during a more humid climate, vegetation increases and soil erosion is less (Krom et al, 2002).…”